Fix bp2build select generation for inter-attribute soong config

variable usage.

There's bug a in the current soong_config_variable handling
implementation where a soong_config_variable sets conditions_default
value for an attr, and a non-conditions_default value for another attr.
This results in the former attr not properly setting the zero value for
the non-conditions_default select key, resulting in the pretty printer
omitting the attribute totally.

The current implementation in this CL ensures that the zero value is set
whenever this happens at the module level. This is seen in
library_linking_strategy_cc_defaults (see comments in code, and the new
tests)

Test: CI
Bug: 198556411

Change-Id: Ibaeb94508c51a7429fb7a08df610cbb5470f76d2